A pillar-shaped via structure in a Cu-polyimide multilayer substrate and its novel fabrication process are described. The process requires that a fine rectangular via conductor be formed by pattern electroplating using a thick positive photoresist. In addition, a flat polyimide dielectric is formed only by the photolithographic process using a photosensitive polyimide precursor. The resulting pillar-shaped via conductor is 30-μm square and 25-μm thick per layer. The area occupied by this via conductor is 25% smaller than that occupied by a conventional via not filled with copper conductor. Its signal line density is twice as high, and the thermal resistance is about 50% less than that of a conventional via. This structure is suitable for high-speed signal transmission in a Cu-polyimide multilayer substrate